ViewHelper: Remove dependency to EditorControls.

[Mugen87]

Related issue: -

Description

ViewHelper has a dependency to EditorControls right now by relying on the control's center property. That makes it a bit inconvenient to us the helper with other controls like OrbitControls. You essentially have to do this:

const helper = new ViewHelper( camera, renderer.domElement );
helper.controls = controls;
helper.controls.center = controls.target; // hack

The PR removes the dependency and introduces a new center property instead. ViewHelper.center can be directly modified by app level code or the center/target property of a control class can be assigned to it (like in editor/js/Viewport.js).